Magnesiowiistite

Figure 5.2 Crystal field spectrum of magnesiowiistite, (Mgo74Fe026)0 (from Goto et al, 1980).

Figure 5.3 3d orbital energy level diagram for Fe2+ ions in the periclase structure (from Bums, 1985a). Observed transitions in the spectrum of magnesiowiistite (fig. 5.2) are indicated. 

In high-pressure spectra of some iron-rich magnesiowiistites, absorption edges attributed to oxygen —> Fe charge transfer transitions shift rapidly into... 

Li, X. Jeanloz, R. (1991a) Effect of iron content on the electrical conductivity of perovskite and magnesiowiistite assemblages at lower mantle conditions. J. Geophys. Res., 96, 6113-20. 

Sherman, D. M. (1991) The high pressure electronic structure of magnesiowiistite (Mg,Fe)0 Applications to the physics and chemistry of the Lower Mantle. J. Geophys. Res., 96, 14299-312. 

Simons, B. (1980) Composition-lattice parameter relationships of the magnesiowiistite solid solution series. Ann. Rept. Geophys. Lab., Yearb. 79, 376-80. 

Three kinds of evidence have been put forward in support of a lower mantle with a different composition from the upper mantle. The first was the apparent lack of a match between the seismic and other geophysical properties observed for the lower mantle, and the laboratory-measured properties of lower mantle minerals (MgSi03-rich perovskite and magnesiowiistite) in an assemblage with the upper mantle composition (meaning, effectively, with the upper mantle s Mg/Si and Mg/Fe ratios). Jackson and Rigden (1998) reinvestigated these issues and conclude that there is no such mismatch (see Chapter 2.02). 

Figure 1 Depth-varying phase proportions in a pyrolite model mantle after the manner of Ringwood (1989), Ita and Stixmde (1992), and Bina (1998h). Phases are (a) ohvine, (fi) wadsleyite, (y) ringwoodite, (opx) orthopyroxene, (cpx) clinopyroxene, (gt-mj) garnet-majorite, (mw) magnesiowiistite, ((Mg,Fe)-pv) ferromagnesian sihcate perovskite, and (Ca-pv) calcium silicate perovskite. Patterned region at base denotes likely heterogeneity near core-mantle boundary.

Perhaps one of the most important consequences of a peridotite composition for the upper mantle is that the phase transitions in olivine that are manifested as seismic discontinuities should exhibit thermally controlled variations in their depth of occurrence that are consistent with the measured Clapeyron slopes (Bina and Helffrich, 1994) of the transitions. In particular, the olivine-wadsleyite transition at 410 km should be deflected upwards in the cold environment of subduction zones while the disproportionation of ringwoodite to silicate perovskite and magnesiowiistite at 660 km should be deflected downwards, thereby locally thickening the transition zone. In anomalously warm regions (such as the environs of mantle plumes as described below), the opposite deflections at 410 and 660 should locally thin the transition zone. The seismically observed topography of 20-60 km on each of the 410 and 660 is consistent with lateral thermal anomalies of 700 K or less (Helffrich, 2000 Helffrich and Wood, 2001). 

Magnesiowiistite/Melt Partitioning MgSiOs Perovskite/Melt Partitioning CaSiOs Perovskite/Melt Partitioning Superphase B—Melt Partitioning... 

Figure 10 Deep mantle phase equilibria (source Bina, 1998). a, and y refer to the different forms of Mg2Si04, opx = orthopyroxene, cpx = clinopyroxene, gt = garnet, mw = magnesiowiistite, Mg-pv = magnesium perovskite, Ca-pv = calcium perovskite, and horizontal lines show the approximate locations of oxide transitions in Si02, FeO, and AI2O3.

However, it is clear that magnesiowiistite is a significant host phase for manganese, chromium, and vanadium and may contribute to the depletions of these elements in the Earth s upper mantle, rather than metal/silicate equilibrium. 

Figure 13 Summary of magnesiowiistite/melt partition coefficients at high pressure (sources Gessmann and Rubie, 1998 Ohtani and Yurimoto, 1996 Agee, 1993 McFarlane et aL, 1994 McFarlane, 1994).

Magnesiowiistite is a significant host phase for transition elements. Although nickel is compatible... 

Frost D. J. and Langenhorst F. (2002) The effect of AI2O3 on Fe—Mg partitioning between magnesiowiistite and magnesium silicate perovskite. Earth Planet. Sci. Lett. 199, 227-241. 

McFarlane E. A., Drake M. J., and Ruble D. C. (1994) Element partitioning between Mg-perovsMte, magnesiowiistite, and silicate melt at conditions of the Earth s mantle. Geochim. Cosmochim. Acta 58, 5161—5172. 

Ohtani E. and Yurimoto H. (1996) Element partitioning between metallic liquid, magnesiowiistite and silicate liquid at 20 GPa and 2,500 °C a SIMS study. Geophys. Res. Lett. 23, 1993-1996. 

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